Method and apparatus for reducing fringe field edge effect development in liquid toner electrophotography

Abstract: A combination developer and toner delivery roller (12) is used to carry a layer of liquid toner (48) to a development nip (46) formed between the conductive surface of roller (12) and the undeveloped, imaged photoconductor surface (44) so as to reduce preexposure of the undeveloped, imaged photoconductor surface (44) to minimize independent fringe field induced edge effect development. The layer of liquid toner (48) is applied to the surface of roller (12) by either contact with an elongated meniscus of liquid toner emanating from elongated slot (22) in toner delivery assembly (14), or by a spray application through a plurality of toner spray nozzles (54).

Description: BACKGROUND OF THE INVENTION Technical Field This invention generally relates to the use of a combination developer and liquid toner delivery drum to deliver liquid toner to a photoconductive surface to reduce unwanted fringe field edge effect toner development, and more particularly to a combination developer and liquid toner delivery roller that carries a film of toner from a supply source to a nip line formed between the combination developer and delivery drum and the photoconductive surface in both charged and discharged area development photoelectric processes. Background Art In both charged area and discharged area development, using a development roller to help establish the correct electromagnetic force fields (EMF) and to regulate the amount of liquid toner transferred to the photoconducting surface is quite common. For purposes of this disclosure, discharged area development is disclosed and used to place the present invention in proper context. However, it should be distinctly understood that the present invention is adaptable for use with charged area development. Also, for purposes of this disclosure, a photoconductive drum is used, although photoconductive belts are also in common use, and the principles of this disclosure apply equally to either drums or belts. In discharged area development, the photoconductor is provided with a uniform charge, either positive or negative. The development roller is provided with a similar charge, as are the particles of toner to be applied to the photo-conductor. The image to be developed, either a dither pattern image or a solid image, is imprinted upon the photoconductor drum. The dither pattern image is usually imaged by means of a laser print engine. A print engine is used to expose portions of the surface of the photoconductor drum to form the imprint of the eventual printed image. Other light sources are used for solid images. Exposure of the surface of the photoconductor to the light reduces the charge on the photoconductive surface material in the exposed area to a level significantly below that of the unexposed areas of the photoconductive surface. In developing the image, two things must occur at the same point. The photoconductor drum, or belt, and a development roller must form an elemental development nip line at a precisely configured, spaced distance apart, and liquid toner, containing the appropriately charged particles, must be injected into the nip in order to develop the image. Toner, as it is being carried through the development nip, which is the elementally aligned closest point of approach between the photoconductive drum and the development roller, finds itself with no place to go except to those exposed or imaged areas that are part of the image to be printed or to be carried out of the nip. This is because the charged toner particles are repelled by the highly charged undeveloped surface areas of the photoconductor drum, and the similarly charged development roller. Thus the charged particles contained in the liquid toner are deposited onto the exposed image areas of the photoconductor for eventual transfer to the paper or other material to produce the printed image. A problem is how to inject the liquid toner into the nip to produce the best possible developed images. The correct amount of toner applied must be uniform along the entire length of the nip line. Too little toner delivered to the nip line will result in underdeveloped printed images; too much in overdevelopment in unimaged areas. Finally, care must be taken to insure that the undeveloped image on the photoconductor drum is not pre-exposed to liquid toner. The development process which occurs within the nip between the development roller and the photoconductor is the result of the electromagnetic forces established by the charged development roller, the unexposed surfaces of the photoconductor, and the exposed surfaces. In the nip, charged particles of toner are repelled away from the development roller and the unexposed, still charged, surface areas of the photoconductor, and on to the exposed surface areas, which are at a substantially reduced charge or perhaps even to ground. If the development roller to photoconductor nip line were to be positioned directly at the bottom of the drum, this development process could be thought of as a vertical development process, or at least one that is normal to the surface of the photoconductor. However, these are not the only electromagnetic fields which are established on the drum as it is being exposed. The difference in the potential between exposed surface areas of the photoconductor at low potential, and adjacent unexposed areas at relatively high potential also establishes electromagnetic force fields on the surface of the drum in the vicinity of the boundaries between exposed and unexposed surfaces. These are called fringe fields. These electromagnetic fields can be thought of conceptually as being horizontal or lateral electromagnetic fields on the surface of the photoconductor, as opposed to the vertical fields within the developer to photoconductor nip. These same fringe fields exist in charged area development. These fringe fields, which are quite strong, are confined to a narrow region at the boundary of each imaged area. They are strong enough to deposit charged particles of toner in narrow regions of the undeveloped, imaged areas of the photoconductor surface adjacent to the boundary with the highly charged undeveloped, unimaged areas. This is called fringe field development or, the edge effect. The existence of the edge effect has been known for a long time. In fact, the edge effect is clearly visible in the very first xerographic image produced by Chester F. Carlson, which was made on October 22, 1938. Even in today's liquid toner electrophotographic printers, if an attempt is made to print using a liquid toner having an insufficient density of charged particles, the toner will be deposited adjacent to the boundaries of the imaged areas first, to the extent that it is possible to print only the outlines of the image with a thin enough density of charged particles in liquid toner. Even with the use of high quality liquid toner having a sufficient density of charged toner particles, the edge effect is still of concern. If the imaged photoconductor surface is pre-exposed to toner before being exposed to the developer nip, the fringe fields will start to develop by deposition of particles of charged toner in the boundary areas of the exposed, imaged areas of the surface. If the undeveloped photoconductor surface is pre-exposed to toner long enough, the edge effect produces undesirable excess toner development along the boundary areas. In the prior art, liquid toner is delivered to the nip line either by spraying it into it or by immersing the nip line in a bath of liquid toner. Both of these prior art methods of delivering liquid toner to the nip line pre expose the imaged photoconductor to liquid toner. With a nozzle to nip toner delivery system, there is overspray of toner onto the imaged photoconductor, and with the nip immersion bath, the fringe fields within the imaged photoconductor surface start developing toner as soon as the photoconductor surface is immersed within the bath. EP-A-0481516 discloses a wet development apparatus for a recording machine. The apparatus includes a development roller disposed in contact or near a latent image carrier and an application head for applying a uniform layer of wet developer to the roller. The upper portion of the developer application head which is made of electroconductive material is kept at an electric potential which is higher than that of the development roller, thus generating an electric field between the upper portion of the developer application head and the developer roller. The wet developer, which has a high concentration of solid, is driven out of the developer outflow slot of the head and uniformerly applied to the peripheral surface of the development roller. The liquid toner is ejected by the developer application head and transferred to the development roller by electrostatic effects. It is the object of the present invention to provide a simplified liquid electrophotography system and a method for delivery of toner to a development nip line in a liquid electrophotography system. This object is achieved by a system according to claim 1 and by a method according to claim 4. BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the present invention will be described in the following with reference to the drawings, in which Fig. 1 illustrates a preferred embodiment of the liquid electrophotography system according to the present invention; Fig. 2 illustrates the system of Fig. 1 during operation; and Fig. 3 illustrates the mechanical structure of a toner delivery assembly in more detail. DETAILED DESCRIPTION OF THE INVENTION A preferred embodiment of the apparatus for reducing fringe field edge effect development of imaged photoconductor areas is shown in schematic format in Figs. 1 and 2. It is intended for use with liquid toner in a liquid toner electrophotography process. The dimensional, voltage, and rotational speed information provided herein are representative of a functionally working discharge area development system, however, it should be apparent to those skilled in the art that a wide range of dimensional, voltage and rotational speed variations will work equally well in other liquid electrophotography applications, including various charged area development processes. Beginning with a combination developer and delivery roller 12, which, in the preferred embodiments, rotates in the counter clockwise direction at a velocity within the range of 100mm/second to 300mm/second. Combination developer and toner delivery roller 12 picks up a uniform thin film of liquid toner from a meniscus of liquid toner forming atop toner delivery slot 22 of toner delivery assembly 14. Combination developer and toner delivery roller 12 carries this uniform thin film of toner on its surface and into development nip 46 formed between the surfaces of combination developer and toner delivery roller 12 and photoconductor surface 44, which for purposes of this disclosure is a drum, but could just as easily be a belt or any other photoconductor surface. Photoconductor surface 44 is, in this first embodiment, traveling in the direction of arrow V 1 and at a velocity of approximately 75mm/second. In all of the preferred embodiments, the liquid toner contains positively charged developer particles within a viscous toner fluid. Combination developer and toner delivery roller 12 is biased to approximately + 400 volts, and undeveloped areas on photoconductor surface 44 are biased to approximately + 600 volts, with laser imaged areas having a reduced voltage bias in the range of + 20 volts to + 80 volts. As is known in the art, given particular rotational surface speeds, properties of particular liquid toners, and voltage biases, there will be a required closely toleranced dimensional spacing at the nip 46, which is identified as D 2 in Fig. 1 between the surface of toner delivery roller 12 and the photoconductor surface 44. In the preferred embodiment, this dimension is within the range of .05mm to .08mm, in order to effectively and completely develop imaged areas on photoconductor surface 44 at nip 46. Combination developer and toner delivery roller 12 is formed of conductive material. Photoconductor surface 44 is a conductive organic compound. As shown in Figs. 1, 2 and 3, there exists a reservoir 34 which holds a quantity of liquid toner 36. Toner pump 28 draws a supply of liquid toner from reservoir 34 through suction line 32 and discharges it through discharge line 30 which interconnects with tubular connector 38 of toner delivery system assembly 14. From there, the supply of liquid toner is pumped into toner delivery plenum 20, which is completely filled with liquid toner during operation. As can be seen in Fig. 3, toner delivery plenum 20 has a sloped bottom surface which facilitates automatic drain out of toner delivery assembly 14 when pump 28 is not operating. As liquid toner overfills toner delivery plenum 20, it eventually fills toner delivery slot 22 formed between toner delivery body 16 and face plate 18, and emanates therethrough to form an elongated convex meniscus of toner atop slot 22 in the top surface area of toner delivery assembly 14 formed of toner pickup surface 24 of toner delivery body 16, and angled surface 42 of face plate 18. Under ideal conditions, the exact quantity of toner that would be required for the electrophotography process should be pumped through toner delivery slot 22. Less than the full amount required to develop imaged areas on photoconductor surface 44 would result in underdeveloped images on photoconductor surface 44, which would be an unacceptable result. To preclude this, excess liquid toner is pumped through slot 22 in a quantity sufficient to insure that the toner layer 48 carried on the surface of combination developer and toner delivery roller 12 is sufficient to fully develop images on photoconductor surface 44. The excess toner 50 flows down toner drain surface 26 of toner delivery body 16 and back into the reservoir of toner 36 found at the bottom of reservoir 34. Toner 48, carried on the surface of combination developer and toner delivery roller 12, is not held in place by means of any EMF field on the conductive surface of toner delivery roller 12, but rather the amount of toner 48 held in a nearly uniform film on the surface of toner delivery roller 12 is dependent upon the viscosity of the liquid toner, its surface tension properties, the adhesion properties between the toner and the metal surface of toner delivery roller 12, the velocity of toner delivery roller 12, and the spacing between the surface of combination developer and toner delivery roller 12 and toner delivery assembly 14 at the nip line formed between the surface of toner deliver roller 12 and toner pickup surface 24 and angled surface 42. In the preferred embodiment, the spacing is shown as dimension D 1 and is approximately 0.7mm. The toner is formed of suspended charged particles in a dielectric liquid, having a viscosity of 1.2 centipoise. Starting generally with the chemical composition of the toner and, its viscosity and surface tension characteristics, the necessary spacing and rotational speeds can be empirically determined to provide for a uniform adequate layer of toner 48 being delivered to nip 46. Under normal imaged area development operations, not all of toner 48 will be used in the development process. Some leftover toner 52 will remain on combination developer and toner delivery roller 12. This is scavenged off of the surface of toner delivery roller 12 by means of cleaning roller 58, which, in the preferred embodiment, is rotating in the direction of arrow V 3 . The direction of rotation of cleaning roller 58 is not critical. It can rotate in either direction as long as there is relative motion between the surfaces of cleaning roller 58 and combination developer and toner delivery roller 12. Scavenged leftover toner 52 drops off of cleaning roller 58 back into the pool of liquid toner 36 found in reservoir 34. Cleaning roller 58, in the preferred embodiment, is formed of a polymer foam. The mechanical structure of toner delivery assembly 14 is shown in more detail in Fig. 3. As can be seen, it is formed of toner delivery body 16, which has formed integral therewith an internal toner delivery plenum 20. The bottom surface of toner delivery plenum 20 is sloped back down to tubular connector 38 so as to automatically drain liquid toner from plenum 20 when pump 28 is not in use. Toner delivery slot 22 is formed with careful attention to its back surface dimensions so that the slot formed between toner delivery body 16 and face plate 18 is of uniform width along its entire length. This insures that a uniform volume of toner is presented to toner delivery roller 12. Face plate 18 is attached to toner delivery body 16 by means of conventional bolts, not shown.

Claims: A liquid electrophotography system using a photoconductor surface (44) having undeveloped imaged areas, and a liquid toner having suspended within it charged toner particles, the liquid electrophotography system comprising: a conductive, charged roller (12) rotatably disposed in juxtaposed, transverse elemental alignment with the photoconductive surface (44) for forming a nip line (46) for operatably developing imaged areas of said photoconductor surface (44) by depositing charged toner particles onto the undeveloped imaged areas of said photoconductor surface (44); means for rotating the surface of said roller (12) having the layer of liquid toner thereon into the nip line (46) formed between the roller and the photoconductive surface; and means for depositing a layer of liquid toner onto the surface of the roller (12) comprising means (42) for forming an elongated row-shaped meniscus of liquid toner; means (20,28) for supplying and continuously replenishing the meniscus of liquid toner with more toner as liquid toner from the meniscus is deposited to the surface of the roller (12); and means for positioning said liquid toner meniscus forming means (42) in elemental juxtaposed alignment with the roller (12), with the meniscus in contact with the surface of said roller (12) wherein a layer of said liquid toner will be deposited onto the surface of the roller (12) as said roller (12) is rotated. The system of claim 1 wherein said means (42) for forming an elongated row shaped meniscus of liquid toner is specified by: a toner delivery body (16) having an elongated slot (22) for the passage of liquid toner (36) therethrough, and plenum means (20) for the receiving, and the passage therethrough to the elongated slot (22), of a supply of liquid toner from the means for supplying and continuously replenishing the meniscus of liquid toner. The system of claims 1 or 2 wherein said means (20, 28) for supplying and continuously replenishing the meniscus of liquid toner with more toner as liquid toner from the meniscus is deposited to the surface of the roller (12) is specified by: a reservoir (34) for holding liquid toner; and means for pumping liquid toner (28), operatively connected to the means (42) for forming an elongated row shaped meniscus of liquid toner, for pumping liquid toner (36) from the reservoir (34) to said means (42) for forming an elongated row shaped meniscus of liquid toner (16). A method for delivery of toner to a development nip line in a liquid electrophotography system having a photoconductor surface (44) having undeveloped imaged areas, a liquid toner having suspended within it charged toner particles, and a conductive, charged roller rotatably disposed in juxtaposed, transverse elemental alignment with the photoconductive surface (44) for forming an electro-photographic development nip line (46) for operatably depositing charged toner particles onto the undeveloped image areas of the photoconductive surface (44), the method comprising the steps of: depositing onto the surface of the roller (12) a layer of said liquid toner by: forming an elongated row-shaped meniscus of liquid toner; positioning said elongated row-shaped meniscus of liquid toner in elemental juxtaposed alignment with the roller (12) with the meniscus in contact with the surface of said roller (12) wherein a layer of said liquid toner will be deposited onto said roller surface as said roller (12) is rotated; and supplying and continuously replenishing the meniscus of liquid toner with more toner as liquid toner from the meniscus is deposited onto the surface of the roller; rotating the surface of said roller (12) having the deposit of liquid toner thereon into the nip line (46) formed between the roller (12) and the photoconductive surface (44). The method of claim 4 wherein the step of supplying and continuously replenishing the meniscus of liquid toner with more toner as liquid toner from the meniscus is deposited to the surface of the roller is specified by: holding a supply of liquid toner (36) in a reservoir (34); and pumping liquid toner from the reservoir into the elongated row-shaped meniscus of liquid toner.

Inventors: BARNES KRISTINA J, GELEYNSE CARL D, BARNES, KRISTINA J., GELEYNSE, CARL D.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1994
Main classification: G03G15
IPC codes: G03G15, G03G13